Most operating systems, including the WINDOWS family of operating systems by MICROSOFT, are priority-based models, meaning that the determination of which processes or threads get executed first are based on a level of importance—or priority—assigned to the processes or threads. For example, a thread dealing with spreadsheet calculation may be assigned a lower priority than a driver interrupt, because it is more important to process a driver interrupt (in the event, say, that a modem just received sixteen more characters) than it is to process the spreadsheet calculation. Part of the priority in this case is that other processes may depend on the fact that other processes depend on receiving the data from the modem.
One of the problems encountered with a priority-based system is that some processes may be preempted by a process having a higher priority. While this is not a problem in some cases, there are times when a pre-empted process may be pre-empted for longer than an acceptable amount of time. Streaming audio and video processes, for example, cannot be pre-empted for too long without experiencing unacceptable delays that result in glitches. Furthermore, if several applications are running that have threads that are assigned a high priority, the streaming media may be held off too long to execute properly.
Multimedia applications in particular have a goal of providing glitch-free operation so a user experiences a technically smooth operation. However, to obtain dependable glitch-free operation in a priority-based system, data must be submitted in large segments. Providing data in large segments causes a problem with high latency periods (the time from rendering the segment, or buffer, until the time the segment is played). High latency is undesirable for multimedia applications.
The high latency problem may be solved by submitting multimedia data in smaller segments. However, when this is done, the multimedia application is subject to interruption by higher priority threads. In a worst case scenario, several higher priority threads can be processed while the multimedia data is held off indefinitely.
A solution that many developers are turning to is to implement a realtime scheduling system, where each thread to be executed is allocated a certain percentage of guaranteed processor time for execution. Use of a realtime scheduler guarantees that a thread won't be delayed indefinitely or for too long to prevent unacceptable execution.
However, use of a realtime scheduling system presents another problem: even if components within an operating system are realtime-based, there are many applications, drivers, hardware, etc., that schedule threads on a priority basis. Therefore, the result is a component that schedules threads on a priority basis to a system component that schedules threads on a realtime basis. If threads are simply passed through to a realtime scheduler as they are received on a priority basis, the same limitations experienced with priority-bases systems will still be seen.
There is a need for a way to run priority-based components, or threads, on a realtime scheduler so that the priority-based component can experience the advantages of a realtime system.